1. Field of the Invention
The present invention relates to systems for treating obstructive sleep apnea by providing positive airway pressure to a patient, in particular, to methods for the device to adjust the treatment pressure in response to breathing events.
2. Description of the Related Art
One major treatment approach for obstructive sleep apnea includes providing breathing gases to the patient throughout the period the patient is asleep. These treatments may collectively be known as positive airway pressure therapy (PAP). Variations on this therapy include having different inspiration and expiration pressures, commonly known as bi-level or bi-PAP, or having continuously adjusting therapy which responds to breathing events.
Self-adjusting or self-titrating PAP systems typically increase the treatment pressure in response to events, and decrease the treatment pressure in response to periods of normal breathing. Events for which the systems increase pressure can include apneas, hypopneas and obstructed breathing. Many different methods have been provided for determining the occurrence of an apnea, a hypopnea or obstructed breathing. For example, our U.S. patent application Ser. No. 11/198,072 describes a system that determines events by analysing a flow signal provided by a flow sensor. The system determines an apnea based on a period where the flow signal indicates a lack of patient breathing. The system determines a hypopnea based on a period of reduced breathing volume. The system determines obstructed breathing on analysis of the discreet energy spectrum of the flow signal of patient breaths. The system modifies the treatment pressure depending on the present pressure and the determined event. When responding to apneas or hypopneas, the described system only increases the pressure if the present treatment pressure is below 10 cmH2O. When responding to flow limitations and obstruction, the system will always increase pressure, subject to an overall maximum allowed pressure.
An upper limit for responding to apneas and hypopneas that are not accompanied by a flow limitation is to recognise that not all events are caused by obstruction and collapsing airways which can be alleviated with increased pressure. Instead, some events, commonly referred to as “central” events, do not respond to increases in pressure. Many CPAP patients suffer from both obstructive events and central events. Although increasing the treatment pressure in response to central events is not usually dangerous, it is believed that it could trigger additional central events or rouse the patient. This is generally thought more likely with higher CPAP pressures, and the likelihood that an event is central is generally thought more likely at higher treatment pressures.
Nonetheless, a defined threshold may not be the optimal solution for deciding whether to increase the pressure in relation to apneas and non flow-limited hypopneas. In some cases, where central apneas are being experienced at lower pressures, this system will allow the CPAP pressure to reach a level that is not required. In other cases, the threshold pressure may be reached with the patient still incurring obstructed apneas that could respond to further increases in pressure.
Methods have been devised to attempt to differentiate between open airway apneas, which are unlikely to respond to increases in PAP pressure and closed airway apneas, which may respond to an increase in therapy pressure. These include analysing the flow in pressure signals to determine the presence of airflow, that is airflow that is generated by the beating heart which can slightly affect the lung volume. Another method involves examining the airway response to a forced oscillation or probing signal. Both these methods add complexity to the system and may not be fully reliable across a wide range of patients.